Spark plugs for generating discharge in gaps between anodes and cathodes are widely used in order to ignite fuel-air mixtures filling combustion spaces of internal combustion engines such as automobile engines.
In the spark plugs, when the gaps between the anodes and cathodes are widened, discharge is not generated if voltages to be applied between the anodes and the cathodes are not heightened. Further, depending on compositions and pressures of the fuel-air mixtures, discharge is generated at unintended timing and the spark plugs may be damaged by arc discharge, thereby causing a problem that the stability of the discharge is deteriorated. Since the compositions and pressures of the fuel-air mixtures are not constant, the deterioration in the stability of the discharge causes deterioration in stability of igniting the fuel-air mixtures.
However, when the gaps between the anodes and the cathodes are not widened, a discharge that spread widely and three-dimensionally is not generated, thereby causing another problem such that combustion efficiency and a combustion speed of the ignition of the fuel-air mixtures are not improved.
In order to solve these problems, a spark plug in Patent Document 1 is provided with an auxiliary electrode (floating electrode 11) in addition to an anode (center electrode 3) and a cathode (outside electrode 6), so that a gap between the anode and the cathode is widened.